The WideLead TO-262 package offers performance
improvements for both inside and outside the package improving the
system efficiency and reducing the long electrical path bottleneck
of the standard TO-262

Over the last 12 months hybrid (HEVs) and
electric vehicles (EVs) seem to have been all the fashion and
indeed the car manufacturers who don’t have an HEV in
their line-up are certainly a shrinking minority. However a far
bigger trend in the automotive market is one that has gone
virtually unnoticed the electrification of the traditional internal
combustion engine driven car. This trend has been far more
significant in terms of the number of vehicles affected and as a
result it could be argued has had a far greater effect on the
reduction of emissions and improving efficiency. Having said that
this is a trend that is not directly marketed by car OEMs but it is
all over new cars. Electric power steering, electric fuel and water
pumps, class D audio and direct fuel injection are all examples of
the electrification of the car and new electric systems that are
lighter, more compact and more fuel efficient than the traditional
solutions.

One area of rapid growth is Electric Power
Steering (EPS), estimates for exactly how much fuel is saved here
vary, but a 5% improvement in miles per gallon is frequently cited
as the benefit of replacing a hydraulic power steering pump that is
in action the whole time the car is running to an electric motor
which just draws a current when needed. Size and power are key
features for EPS systems and to achieve good cooling manufacturers
frequently take the step of mounting the power electronics on a
substrate with much better thermal performance than a standard PCB
material. However having the power and control boards separate
means that a leaded component (like a standard TO-262 packaged
MOSFET) needs to be used to form the connection between the two
(see Fig. 1).

Fig. 1: Typical power and control board
assembly using through hole power electronic components.

However one of the main weaknesses in doing such
an assembly is the relatively long electrical path and associated
resistance of the leads. To combat this International Rectifier
recently released a series of automotive grade parts in a new
WideLead TO-262 package. Table 1 shows the performance of the two
new parts compared with the same MOSFET in a traditional TO-262
package.

As shown in Table
1, for both IR’s 1324 and 3004 MOSFET families,
the maximum RDS(on) value can be
reduced by around 20% by introducing the new WideLead TO-262
package. On the other hand, wider lead means more areas to fit the
internal wirebonds at MOSFET source terminal, the combined effects
of both lower RDS(on) and improved wire bonding inside the package
results in an approximately 30% increase of the maximum drain
current rating: from 195 A for standard TO-262 package on the
market to 240 A for WideLead counterpart.

Table 1: Comparison of performance
differences between the standard TO-262 and WideLead
packages.

It is important to point out that the Rds(on) on
TO-262 packages in specified very close to the package —
the RDS(on) does not include any resistance due to the leads, and
we already know that the silicon resistance is significantly
reduced for WideLead TO-262 package. However, In order to optimize
the system performance mentioned on Fig. 1, the associated
resistances from the whole electrical path need to be considered:
both silicon resistance inside the package and lead resistance.
Another important benefit of WideLead is that lead resistance can
be reduced significantly as well. Figure
2a shows the outline of the traditional TO-262 package, and
Fig. 2b demonstrates the new
WideLead To-262:

Let’s take AUIRF1324WL as an example.
On the datasheet, the RDS(on) is
stated at 1.65 mΩ max. However, in the application the
lead resistance becomes significant, the total resistance of the
drain and source leads on a traditional TO-262 is typically around
1 mΩ! So assuming the customer uses the full length of the
leads in the application the resistance might increase form 1.65 to
2.65 mΩ — a total 60% resistance increase!
However, if we switch to WideLead package, 50% lead resistance
reduction can be achieved thanks to the wider lead, which means the
total resistance could be reduced from 2.65 mΩ to only
2.15 mΩ. As a result, lower condition loss and in turn,
better system efficiency is achievable. On the other hand, the body
and form factor of the WideLead TO-262 package remains the same as
the traditional TO-262, so no major mechanical redesign is needed
when switching to the WideLead package.

Fig. 2a: Traditional TO-262 package

Fig. 2b: The new WideLead TO-262

Figure 3
demonstrated the benefits of the WideLead package at a system
level; this chart plots the temperature of the leads against DC
current for both the standard TO-262 package (AUIRF1324L) and
WideLead TO-262 package (AUIRF1324WL), in both cases the silicon
inside the package is identical. At 60-A drain current level, the
WideLead was 39% cooler than standard TO-262, by having less heat
generated, the cooling arrangements can be downsized or perhaps a
lower grade of PCB material could be used. Reduced heat dissipation
also translates to reduced reliability concerns, which is
definitely a key benefit considering the stringent automotive
reliability requirement.

We could look at Fig. 3 in a different way: For
a given operating temperature up to 30% higher current can be
achieved with the same silicon, which means more load capability
and higher power density. Either way improved performance can be
achieved with possible system cost reduction as the package
performance becomes less of a limitation on the semiconductor
inside.

Fig. 3: Difference in lead temperature
between a standard TO-262 package and the new WideLead TO-262
across a range of currents.

With the performance improvement from both
inside and outside the package, the WideLead TO-262 package is a
strong candidate for improving the system efficiency to meet the
market needs, at the same time it improves the system RDS(on) performance and reduce the long
electrical path bottleneck for standard TO-262. With the
innovations like the WideLead package, system improvement can be
easily achieved from both thermal and cost prospective.
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